There are methods presently available for diagnosing coronary artery disease, each having its own advantages and disadvantages. The most definitive procedure is coronary angiography wherein a radio opaque dye is injected into the coronary arteries However, this procedure is invasive and presents a risk to the patient, and cannot be used routinely or for medical screening. Another procedure involves the analysis of the electrical activity of the heart (the electrocardiogram or ECG) while under stress, to identify cardiac degradation resulting from an inadequate blood supply. While this method is noninvasive, it attempts to identify an effect of the disease, and therefore, it is not always reliable and cannot identify the disease in its early stages.
It has been found that coronary stenosis generates an auditory component due to turbulent blood flow in the partially occluded coronary arteries. Normally this auditory component is not present in a healthy patient. However, efforts in the past to detect this auditory component have met with limited success. See J. Semmlow, W. Welkowitz, J. Kostis, and J. Mackenzie, "Coronary Artery Disease - Correlates Between Diastolic Auditory Characteristics and Coronary Artery Stenoses." IEEE Trans. Biomed. Engr., Vol. BME-30, No. 2, Feb. 1983. The auditory component associated with coronary stenosis is very weak and heavily contaminated with noise. Its detection by way of an acoustic transducer suffers from considerable attenuation due to the intervening heart tissue, the chest wall, and other compressible tissues such as lung or fat. In addition, its detection may be masked by the comparatively loud heart valve sounds as well as other naturally occurring body sounds or external ambient noise.
Previous efforts for isolating and detecting this low level auditory component have failed to provide results of sufficient reliability to accurately predict coronary stenosis. Previous techniques using traditional spectral methods such as the Fast Fourier Transform (FFT) could not provide sufficient resolution under such low signal to noise conditions. See M. Ortiz, J. Semmlow, J. Kostis and W. Welkowitz, "Coronary Artery Disease: Noninvasive Diagnosis." 40th ACEMB Proceedings, Sept. 1987.
The present invention noninvasively records and analyzes the heart sounds from a patient in order to reliably detect, among the other heart sounds, the presence of the auditory component associated with coronary stenosis. In the past, acoustic based systems have been employed to detect partial occlusions in other arteries such as the carotid or femoral arteries. However, these systems were only effective for detecting the relatively high level sounds from those arteries found in close proximity to the skin. These systems were not capable of reliably detecting the highly attenuated auditory component associated with partially occluded coronary arteries.
Also, in the past, phonocardiographic applications of parametric, or model-based, spectral analysis methods were utilized in the analysis of valve sounds for diagnosing valvular disorders However, the analysis of these valve sounds again involved signal levels many times greater, and with correspondingly larger signal-to-noise ratios, than the auditory component associated with coronary stenosis which is sought to be identified in the present invention.
The present invention includes isolating the diastolic segment of a heart sound recording in order to more accurately analyze the auditory component of interest. When analyzing heart sounds in the past, a diastolic time "window" was positioned using either the judgment of a human operator or automatic placement techniques. However, these previous windowing techniques often resulted in a misplaced window, or lacked editing features which would detect and reject artifacts due to breath sounds, stomach growls, external ambient noise, etc. Not surprisingly, such window misplacement and/or presence of artifacts often led to erroneous results, thus providing unreliable diagnostic information.
Additionally, the detection of heart sounds have been hindered in the past by the detrimental effects of existing acoustic transducers. For example, mass loading by the acoustic transducer on the chest wall tended to attenuate and distort the recorded heart sounds. This attenuation typically increased as the load increased. To further complicate matters, the detrimental effect of loading became greater at the higher frequencies. Therefore, the detection of the low level sounds associated with coronary stenosis posed a serious problem, as the signals of interest have frequency components as high as 1.5 kHz. With the effect of mass loading, these highly attenuated signals could easily fall below the noise level and thus become undetectable.
Another obstacle associated with the acoustic transducer was that, typically, the prior art transducers and cardiac microphones had a flat frequency response with a high resonant frequency. This high resonance resulted in reduced sensitivity in the region of 300-1200 Hz, a region which has been found to be important in detecting sounds due to coronary artery stenosis.
Thus, there existed in the art a strong need for a noninvasive method and system for reliably diagnosing coronary artery disease based on detected heart sounds, even in the early stages of the disease. In addition, there existed a need for a method and system capable of reliable identification of a low level auditory component associated with coronary artery disease, especially in the presence of noise. Furthermore, there existed a need for a diastolic windowing technique for automatically isolating the diastolic segment of a heart sound recording and editing the segment for artifacts. In addition, there existed in the art a strong need for a low-mass acoustic transducer of sufficient sensitivity for detecting the low level auditory component associated with coronary artery disease.